Thermoplastic polyamides, such as nylon 6,6, are a class of materials which possess a good balance of properties comprising strength and stiffness which make them useful as structural materials. However, for a particular application, a thermoplastic polyamide may not offer the combination of properties desired, and therefore, means to correct this deficiency are of interest.
One major deficiency of thermoplastic polyamides is their poor resistance to impact, especially when dry. A particularly appealing route to achieving improved impact resistance in a thermoplastic is by blending it with another polymer. It is well known that stiff plastics can often be impact modified by addition of an immiscible low modulus rubber. However, in general, physical blending of polymers has not been a successful route to toughen thermoplastic polyamides. This is due to the poor adhesion immiscible polymers typically exhibit with each other. As a result, interfaces between blend component domains represent areas of severe weaknesses, providing natural flows which result in facile mechanical failure.
It is well known to those skilled in the art that hydrogenated block copolymers of styrene and butadiene possess many of the properties which are required for impact modification of plastics. They have a low glass transition, low modulus rubber phase which is required for toughening. Because they contain little unsaturation, they can be blended with high temperature processing temperature plastics without degrading. In addition, they are unique compared to other rubbers in that they contain blocks which are micorphase separated over application and processing conditions. This microphase separation results in physical crosslinking, causing elasticity in the solid and molten states. Such an internal strength mechanism is often required to achieve toughness in the application of plastic impact modification. In addition, melt elasticity of the block copolymer during processing can, under the right conditions, enable it to be finely dispersed with another polymer in a stable interpenetrating co-continuous phase structure. A stable, fine dispersion is desirable in a rubber modified plastic.
Proof that hydrogenated block copolymers of styrene and butadiene are useful plastic impact modifiers can be seen in their widespread use for modifying polyolefins and polystyrene. For these blends, interfacial adhesion is great enough to achieve toughening.
Although the hydrogenated block copolymers do have many of the characteristics required for plastic impact modification, they are deficient in modifying many materials which are dissimilar in structure to styrene or hydrogenated butadiene. Blends of the hydrogenated block copolymer with dissimilar plastics are often not tough due to a lack of interfacial adhesion.
A route to achieve interfacial adhesion between the hydrogenated block copolymer and a dissimilar material is by chemically attaching to the block copolymer functional moieties which interact with the dissimilar material. Such interactions include chemical reaction, hydrogen bonding, and dipole-dipole interactions.
Epstein in U.S. Pat. No. 4,174,358 discloses a broad range of low modulus polyamide modifiers. However, this patent does not disclose or suggest the use of modified block copolymers of styrene and butadiene.
It has been previously proposed to increase the impact strength of polyamides by addition of a modified block copolymer. Hergenrother et al in U.S. Pat. No. 4,427,828 and Shiraki et al in International Kokai Application No. W083/00492 disclose blends of thermoplastic polyamide with a modified block copolymer. Specifically, the block copolymer is a partially hydrogenated monovinyl aryl/conjugated diene to which is attached anhydride moieites by the so-called "ENE reaction". Such modified block copolymers contain functional moieties only in the diene block, unlike the present invention. In addition, such modified block copolymers are deficient because the ENE reaction depends on unsaturation in the base polymer for reaction sites. A reasonable amount of residual unsatauration must be present in order to obtain an advantageous degree of functional moeities onto the base polymer. Since the ENE reaction cannot be carried out so that all double bonds on the base polymer are scavenged, the result of such a process is a modified block copolymer which contains too high a level of unsatauration for successful impact modification of high processing temperature thermoplastic polyamides.
The `ENE` process as described in the prior art results in a modified polymer product which is substituted at a position on the polymer backbone which is allylic to the double bond. The reaction can be shown for maleic anhydride as follows: ##STR1## wherein (a) represents addition across a double bond in the main chain of the base polymer and (b) represents addition across a double bond occuring in a side chain. After addition and isomerization the substitution is positioned on a carbon allylic to the double bond.
The allylically substituted polymers are prone to thermal degradation due to their thermal instability. It is known in the art that allylic substituents can undergo what has been referred to as a retro-ENE reaction, see B. C. Trivedi, B. M. Culbertson, Maleic Anhydride, (Plenum Press, New York, 1982) pp. 172-173.
Further, because the ENE reaction requires a reasonable amount of unsaturation in the precursor base polymer, as discussed previously, the resulting functionalized copolymer product will have a significant amount of residual unsaturation and will be inherently unstable to oxidation.